1. Field of the Invention
The present invention relates generally to a fan motor power-off automatic braking circuit, and more particularly to a fan motor power-off automatic braking circuit, which has automatic braking function.
2. Description of the Related Art
In the recent years, a microcontroller unit (MCU) is often associated with a power electronic switch driving circuit to design an electronic product in this field. However, when operating the electronic product, the digital microcontroller and the power electronic switch are often operated at a high frequency to make the electronic product generate high heat. Therefore, it has become a quite critical design issue how to quickly dissipate the heat of the electronic product. At the present time, the most often seen heat dissipation means is a fan system, which operates to forcedly carry away the heat of the electronic product. However, in consideration of the control of the fan and the security in operation of a user, the fan system is generally combined with a braking circuit, whereby when the fan system is powered off, the braking circuit can stop the fan system from operating. This can ensure the precision of the control of the fan and the operational safety of the user.
Please refer to FIG. 1, which is a circuit diagram of a conventional fan braking circuit 1. The conventional fan braking circuit 1 includes a bridge driver 11, a controller 13, a braking circuit 14 and a motor winding L. An input power Vin is connected to the bridge driver 11, the controller 13 and the braking circuit 14 to provide necessary power for these components. The bridge driver 11 is composed of two upper arm switches M1, M2 and two lower arm switches M3, M4. When the fan is powered off, by means of the control of the braking circuit 14, a braking effect is provided for the fan so as to immediately stop the fan from operating. However, in the conventional fan braking circuit 1, it is necessary for the controller 13 to generate pulse width modulation (PWM) signal to the two upper arm switches M1, M2 to make split control instead of generate pulse width modulation (PWM) signal to the two lower arm switches M3, M4 to make split control. In this case, the temperature of the two upper arm switches M1, M2 and the controlling resistor will rise. Therefore, it is necessary to increase the number of the electronic components (such as the resistors) to solve the problem of temperature rise. Moreover, the two lower arm switches M3, M4 are electrically connected with the braking circuit 14. As a result, the cost will be increased due to the arrangement of the braking circuit 14. Also, the complexity of connection between the circuits will be increased.